The present invention relates to the geometry of encapsulant materials in light emitting diodes (LEDs) and particularly to the light pattern emitted from surface mount side view LEDs that produce white light.
Light emitting diodes represent a class of semiconductor materials in which the application of current across a p-n junction drives recombinations between electrons and holes with at least some of the recombinations generating photons. In accordance with well-understood principles of electronics and physics, the wavelength (and thus the frequency) of the photons is based upon the energy change of the recombination. In turn, the energy is defined or constrained by the bandgap of the semiconductor materials; i.e., the energy difference between the material's valence band and its conduction band.
As a result, the color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy red and yellow portions of the visible spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue and violet portions of the visible spectrum as well as in the ultraviolet portions of the electromagnetic spectrum.
In some applications, an LED is more useful when its output is moderated or converted to a different color. In particular, as the availability of blue-emitting LEDs has greatly increased, the use of yellow-emitting phosphors that convert the blue photons has likewise increased. Specifically, the combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as backlighting for color displays. In such devices (e.g., flat computer screens, personal digital assistants, and cell phones), the blue LED and yellow phosphor produce white light which is then distributed in some fashion to illuminate the color elements (often formed by liquid crystals, “LCDs”).
In the present application, the term “white light” is used in a general sense. Those familiar with the generation of colors and of color perception by the human eye will recognize that particular blends of frequencies can be defined as “white” for precise purposes. Although some of the diodes described herein can produce such precise output, the term “white” is used somewhat more broadly herein and includes light that different individuals or detectors would perceive as having a slight tint toward, for example, yellow or blue.
In many conventional applications, the light emitting diode (which in its basic semiconductor structure is typically referred to as the chip), is packaged for its intended use. As used herein, the term package typically refers to the placement of the semiconductor chip on an appropriate electrical structure (sometimes as simple as a small piece of metal) along with a plastic lens (resin, epoxy, encapsulant) that provides some physical protection to the diode and can optically direct the light output.
In many conventional applications, the lens is at least partially formed of a hemisphere. An example is the classic TI ¾ package which is widely recognized and is incorporated in a large number of LED applications.
More recently, light emitting diodes are being used for illumination purposes. In particular, LEDs that can produce white light are used for back lighting flat panel displays (computer screens, personal digital assistants, cellular telephones) via some other type of device (typically a liquid crystal) to generate or display color. In many of these applications, the LEDs are mounted perpendicular to the face of the relevant screen. In this orientation, and instead of being directed at the rear of the screen itself, the LEDs are directed towards the edges of a light guide-often a planar piece of plastic-so that when the light from the LEDs enters the edge of the light guide, the light guide in turn redirects it perpendicularly towards the plane of the display screen.
Light emitting diodes packaged for this purpose are referred to as side view surface mount LEDs or side lookers. Many surface mount side view diodes incorporate a concave meniscus within their housing. A concave meniscus can tend to focus light and yield a higher intensity, but at a cost of overall flux. Nevertheless, a concave meniscus also protects the encapsulant from mechanical damage (the encapsulant is generally more fragile than other portions of the package).
Side mount surface view LEDs that emit white light typically do so by incorporating a blue-emitting LED chip with a yellow-emitting phosphor. The blue light from the chip excites the phosphor to emit yellow light. This produces a combination of blue and yellow frequencies that together generate an appropriate hue of white light.
It has been discovered, however, that when a concave conventional meniscus is used in a side view surface mount LED in combination with certain higher brightness diodes, the concave shape of the encapsulant becomes a disadvantage because it may reduce both flux and color uniformity.
These disadvantages are particularly noticeable for chips with far field profiles that are more Lambertian than typical LEDs. Lambertian refers to the degree to which a surface adheres to the Lambert cosine law which states that the reflected or transmitted luminous intensity (flux) in any direction from an element of a perfectly diffusing surface varies as the cosine of the angle between that direction and the normal vector of the surface. The Lambert cosine law is often expressed as the formula N=NOcosA, where N is the radiant intensity, NO is the radiance normal to the emitting surface and A is the angle between the viewing direction and the normal to the emitting surface.
In practical terms, the emission from a Lambertian emitter is more uniform than that of a less-Lambertian or non-Lambertian emitter.
Accordingly, a concave meniscus tends to reduce or eliminate the far field advantages of those LEDs that produce nearly Lambertian far field patterns.
Accordingly, a need exists to complement the brightness and far field characteristics of high quality diodes with an appropriate lens or encapsulant shape that enhances, rather than hinders, the light output for the intended purpose.